Abstract
The operation of the BIS monitor remains undescribed, despite 20 years of clinical use and 3000 academic articles. The core algorithmic software (the BIS Engine) can be retrieved from the motherboard of the A-2000 monitor in binary form through forensic disassembly using debugging interfaces left in place by the original designers, opening the possibility of executing the BIS algorithms on contemporary computers through emulation. Three steps were required for emulation. Firstly, the monitor input stage monitor was disassembled to determine how EEG signals can be compatibly presented to the Engine. Secondly, the Digital Signal Processor on which the Engine executes was recreated in software. Thirdly, the Engine code was patched, allowing execution separated from monitor hardware. Code performance under noise load was evaluated. EEG signals and BIS variables were obtained from a 13-year-old child in normal physiological sleep using a modern BIS monitor. BIS values in sleeping children exhibit a wide dynamic range, including values nominally associated with clinical anesthesia, providing a risk-free technique to obtain empirical EEG data that broadly exercise the algorithms. Emulation demonstrated a correlation coefficient of R = 0.943, consistent with correlations between official Engine iterations. Additive white noise in the EEG caused a progressive lifting and flattening of BIS values. Emulation replicates BIS Engine behavior, allowing calculation upon existing EEG datasets or signals from other, potentially remote or wireless, devices. Emulation provides advantages for elucidating the mathematical expression of the algorithms, which remain important as practical constraints on any hypothetical mechanism of action of anesthetics.